I just found your post. I have been confronted with similar situations from the other side of the river.
OK, so let's imagine the worst. You disappoint your friend. Reproaches etc... Imagine it. Who's gonna die ? Nobody! You will have made an experience that has good sides whatever the outcome.
Why be so afraid to disappoint ? We all do disappoint. That's allright. The important thing is how we deal with it.
I appreciate it a lot when a friend does something annoying and comes to me to say sorry. If he or she had been perfect, I would never have known that he/she can say : "I'm sorry". Besides, I would have felt pretty dumb next to a perfect friend. Nobody wants a perfect friend. Trust me on that one.
And no, you don't have to be there 100%. You have your needs. Friends understand that. Be there in the tough moments.
You are unfair with yourself. Give yourself a break!